For the production of copper oxide ultrafine particles having a primary particle diameter of less than 100 nm, there has usually been employed a method of protecting the surface of ultrafine particles with a surfactant or a specific three-dimensionally bulky organic compound in order to inhibit excessive increase in the diameter of particles produced by the reaction. In general, according to such production method, the copper oxide ultrafine particles are obtained in the suspended state in the form of a colloid in the reaction solution, and hence a high-speed centrifuging step is necessary for separating the particles as a solid matter from the reaction solution to remove impurities or the like.
Although an explanation will be made based on particular cuprous oxide ultrafine particles, the present invention is however not limited to cuprous oxide ultrafine particles, but can also be similarly applied to other copper oxides.
For example, “Chinese Science Bulletin” (1994, 39, 14-18) discloses that cuprous oxide ultrafine particles having a primary particle diameter of 5-10 nm, the surface of which is covered with dodecylbenzenesulfonic acid, are obtained by dispersing in toluene an aqueous copper acetate solution together with dodecylbenzenesulfonic acid as a surfactant and then reducing the copper acetate (method 1). This method is called the micro-emulsion method which comprises producing microfine water drops having a diameter of several nanometers to several tens of nanometers in an oil layer of toluene and reducing the copper acetate present in the microfine water drops to obtain cuprous oxide. The size of the resulting cuprous oxide particles is of about microfine droplets, and the surface of the fine particles is covered with a surfactant to stabilize the particles.
The cuprous oxide ultrafine particles obtained by this method are in the state of floating in the form of a colloid in water or in the oil layer, and a centrifuging step is necessary for removing impurities in the liquid and separating the ultrafine particles as a solid matter from the solution. However, it is not easy to separate ultrafine particles of less than 100 nm in diameter by centrifugation, and it is generally necessary to use an ultracentrifuge which requires operations to reduce air resistance by keeping a rotating atmosphere or of rotor under reduced pressure. Therefore, productivity is lowered and thus the method cannot be employed for industrial uses requiring mass-production.
On the other hand, “Journal of American Chemical Society” (1999, 121, 11595-11596) discloses that a precipitate of cuprous oxide ultrafine particles having an average primary particle diameter of about 7 nm and covered with a surfactant of one or both of octylamine and hexadecylamine is obtained by pouring an octylamine solution containing a specific organic copper compound into hexadecylamine heated to 250° C. and stopping the heating when the temperature reaches 230° C., followed by cooling (method 2). It is supposed that in this method, amino groups having a strong coordination ability coordinate to the surface of the cuprous oxide particles at the beginning of formation of the particles to inhibit increase of particle diameter of the cuprous oxide.
This method has the feature that the cuprous oxide ultrafine particles are obtained not in the colloidal state in the reaction solution, but in the state of a precipitate, and it requires no centrifugation and hence has the advantage that the particles can be easily recovered. Furthermore, the precipitate per se is a soft agglomerate comprising weakly agglomerated cuprous oxide ultrafine particles whose surface is covered with an amino group-containing organic material, and a colloidal solution of cuprous oxide ultrafine particles can be obtained by redispersing the agglomerate in a suitable dispersion medium such as toluene. However, because these cuprous oxide ultrafine particles have an insulating organic compound of large molecular weight on their surface, they have a problem of being inferior in electric conductivity when they are used as an electrically conductive filler.
On the other hand, there has been known a method for producing cuprous oxide ultrafine particles which do not have a special surfactant or a bulky organic compound on the surface.
“Angewandte Chemie International edition” (2001, No. 40, Vol. 2, p359) discloses that cuprous oxide ultrafine particles having a particle size distribution of 30-200 nm are obtained by dissolving acetylacetonatocopper complex in a polyhydric alcohol and adding thereto a small amount of water, followed by heating to 190° C. (method 3). The cuprous oxide ultrafine particles obtained by this method tend to be larger in particle diameter as compared with those which have a surfactant or a bulky organic compound. Moreover, because the resulting particles have a high monodispersibility and are obtained as a colloidal dispersion, it is necessary to carry out centrifugation in order to remove by-products and separate cuprous oxide ultrafine particles as a solid matter. As such, since the centrifugation operations require labor and time as mentioned above, there has been a problem that the method can hardly be applied to industrial uses which require mass-production.
“Journal of Colloid and Interface Science” (243, 85-89, 2001) discloses a method for producing cuprous oxide ultrafine particles by adding hydrazine to an aqueous alkaline solution of copper sulfate to which a small amount of a polyhydric alcohol is added as an additive (method 4). The cuprous oxide ultrafine particles obtained by this method are preferred because they have a small primary particle diameter of 9-30 nm. And, they further have an advantage that a precipitate of 200 nm-1 μm in secondary particle diameter is produced, and hence the particles can be easily separated from the reaction solution. However, the precipitate obtained is a hard agglomerate comprising secondary particles formed by strong agglomeration of the primary particles, and this precipitate is difficult to redisperse in a dispersion medium. Therefore, a colloidal solution in which the cuprous oxide ultrafine particles are in the colloidal state in the dispersion medium cannot be prepared using the resulting particles.
On the other hand, “Zeitschrift fur anorganische und allgemeine Chemie” (Bd. 224, 107-112 (1935)) discloses that a precipitate of cuprous oxide particles is obtained by adding a 20% aqueous hydrazine solution to a concentrated aqueous copper acetate solution (method 5). However, this literature is silent on amounts of copper acetate and hydrazine as starting materials and only describes that when hydrazine is added in an excessive amount, copper acetate is reduced to metallic copper, and furthermore it does not describe particle diameter of the resulting cuprous oxide.
Summarizing the above methods for producing cuprous oxide ultrafine particles, the cuprous oxide fine particles are obtained (1) in the state of being dispersed in the form of a colloid in the reaction solution (the method 1 and the method 3) and (2) as an agglomerated precipitate (the method 2 and the method 4), and the case (2) is superior from the viewpoint of handleability of the particles. However, a precipitate of the cuprous oxide ultrafine particles obtained by the method 4 have the disadvantage that the precipitate is a hard agglomerate which cannot be redispersed and can hardly be redispersed in a dispersion medium. On the other hand, a precipitate of the cuprous oxide ultrafine particles obtained by the method 2 have the advantage that a colloidal dispersion having the desired composition can be easily prepared by redispersing them in a dispersion medium, but have the problems that the particles have an insulating surfactant on the surface and the actual state of the resulting particles is a composite of cuprous oxide and surfactant, which is difficult to use, for example, as electrically conductive fillers or the like for obtaining copper films by firing.
An object of the present invention is to provide a soft agglomerate of copper oxide ultrafine particles which comprises copper oxide ultrafine particles having an average primary particle diameter of not more than 100 nm and can be redispersed in a dispersion medium and a method for producing the same. Another object is to provide a method for producing a colloidal dispersion in which copper oxide ultrafine particles are dispersed.